The present invention is directed to a dry film photoresist. More specifically, the present invention is directed to a dry film photoresist containing a polymer having unsaturated groups and that generates a free radical.
A dry film photoresist may be either positive working or negative working. Negative working photoresists exposed to activating radiation polymerize or cross-link in a reaction between a photoactive compound and polymerizable agents of the photoresist composition. Consequently, the exposed coating portions are rendered less soluble in a developer solution than unexposed portions. For positive working photoresists, exposed portions are rendered more soluble in a developer solution while areas not exposed remain comparatively less developer soluble.
Dry film photoresists also may be primary photoimaging resists or secondary photoimaging resists. Primary photoresists are used to form temporary coatings on substrates. Secondary photoresists are hardenable and form permanent layers, e.g., solder masks. Photoresists are used to make printed circuits, printing plates, solder masks and the like. Photoresists have various requisites such as etching resistance, heat resistance, adhesion and developable in developer solutions such as alkaline solutions.
Dry film photoresists include at least a resin binder, a cross-linking monomer or oligomer and a photoinitiator. A wide variety of polymeric binders may be employed in dry film photoresists. Such polymeric binders may include, as polymerized components, one or more acid functional monomers such as acrylic acid or methacrylic acid. Polymeric binders take up space in the photoresist and are passively linked to the cross-linking monomers or oligomers. Photoinitiators initiate the cross-linking reaction between the cross-linking monomers or oligomers upon exposure to light. Other additives included in photoresists are anti-striation reagents, plasticizers, speed enhancers, surfactants, fillers, and dyes.
Dry film photoresists may be laminated to a substrate. Such dry film photoresists are particularly suitable for use in printed wiring board manufacture. One problem with many dry film photoresists is that they are difficult to strip from electrolytically plated circuit boards using conventional alkaline aqueous stripping solutions, e.g. 3% sodium hydroxide solutions. If the photoresist is not completely stripped and removed, ragged metal circuit lines may result after etching which may cause short-circuiting of the board.
Many manufacturers use organic-based (amine- or organic solvent-containing) alkaline stripping solutions that produce a smaller stripped particle to facilitate stripping. While the organic strippers, e.g., solutions containing trimethylamine or tetramethylammonium hydroxide, remove the photoresist, such strippers are expensive relative to alkaline aqueous strippers (sodium hydroxide and potassium hydroxide), and have more waste treatment and environmental concerns associated with them. Further, due to emphasis in the industry on reducing solvent emissions in the workplace, solvent-strippable photoresists are much less desirable than the aqueous-strippable.
Instability of a dry film photoresist composition results in a short shelf life. Instability may result from the cross-linking monomers included in the photoresist composition. Prior to exposure of a photoresist composition to actinic radiation, each monomer is a potential reactant with another monomer. If not properly stored or when the photoresist is prematurely exposed to a radiation source, the monomers may prematurely react, thus spoiling the composition and reducing shelf life. Also, after exposure of the photoresist composition to actinic radiation, a fair proportion of the cross-linking monomers may not react. Thus, improper curing of the photoresist may occur resulting in brittle or poorly chemically resistant photoresist.
Additionally, cross-linking monomers or oligomers may precipitate out of the dry film during storage. This phenomena is known as cold flow. Dry film is often stored in rolls. When the cross-linking agents precipitate out and then dry along the sides of the roll, a seal of dry monomers is formed that prevents the dry film from being unrolled prior to use.
Another problem associated with many dry film photoreisists is the build-up of organic sucm and residue from uncured photoresist. Such organic scum and residue may deposit on various articles and apparatus during the manufacture of products made using photoresists such as printed wiring boards, developer solutions and developer apparatus. Much of the organic scum and residue is caused by unsaturated monomers and oligomers such as (meth)acrylate-based compounds and photoactive agents having numerous aromatic groups. Examples of such photoactive agents that may form part of the scum and residue include, but are not limited to, imidazole dimmers, benzophenones, acetophenones, anthraquinones, naphthaquinones, and triazine-based compounds. Such contaminants are not readily water-soluble or water-dispersable after they form residues in solution or deposit on an article or apparatus. As dissolved photoresists build up in solution (developer loading) insoluble organic materials begin to form in the developing tank eventually forming scum or residue. Presence of anti-foam additives (added to developer solutions to minimize foaming) greatly increases the tendency for residue and scum to form. As the level of scum builds chances increase for a redeposit of the scum and residue onto the developed circuit board. Such redeposited residues cause a retardation of etching solution (etching chemistries have difficulty penetrating organic residues). Where etch is retarded, circuit shorts form causing a defective circuit board. In addition to increasing potential for defective circuit boards, the residue and scum also make cleaning equipment difficult, thus increasing maintenance time and cost.
U.S. Pat. No. 5,945,489 and U.S. Pat. No. 6,025,410 both to Moy et al. (also see “Novel Resins That Cure Without Added Photoinitiator” by Sheridan et al. Chemistry III-New Chemistry, RadTech 2002, pages 462-474 (Technical Conference Proceedings)) disclose photosensitive oligomers that may be cross-linked without added photoinitiator. The patents disclose that a Michael addition of acetoacetate donors to multifunctional acrylate receptor compounds yields polyesters with reactive pendent acrylate groups, which may be cross-linked in a subsequent curing reaction. The patents state that pendent methyl ketone substituents serve as an internal photoinitiator. Upon exposure to UV radiation, an acyl radical with the methyl substituent is believed to be formed which acts as a photoinitiator, thus photoinitiators are not added to compositions containing the oligomers. Such oligomers are liquid oligomers, which may be employed as decorative coatings on wood and metal substrates. Odor generated from unreacted photoinitiators and skin absorption of unreacted photoinitiators is avoided, thus compositions containing such oliogmers may be employed in materials that include medical and food contact applications. However, such oligomers are not believed to be suitable for use in photoresists because they are not alkali developable, and are not photosensitive at wavelengths greater than 320 nm. Accordingly, the oligomers of Moy et al. are limited in their applications.
Accordingly, there is a need for a dry film photoresist that eliminates or at least reduces the foregoing problems.